7. Project Summary/Abstract The mechanism of DNA packaging for double-stranded DNA viruses will be studied in the Bacillus subtilis bacteriophage ?29, the most efficient in vitro viral packaging system known. Using an integrated genetic, biochemical and structural approach, we will characterize protein conformational change and movement in the transiently assembled packaging motor during DNA encapsidation. The mechanism of packaging in ?29 will serve as a model for animal virus packaging in the analogous herpesvirus and adenovirus systems, and aid in the search for new antiviral therapies. Due to similarities between the ?29 ATPase and other ring translocases, insights gained from the study of ?29 packaging will also provide insight into the basic principles of macromolecular motor function in higher organisms. To interrogate the mechanism of DNA packaging we will: complete and validate the atomic structure of the ?29 packaging motor using X-ray crystallography and cryoEM analysis (Aim 1); elucidate the mechanisms of force generation and intersubunit coordination during DNA translocation in a complex motor using integrated structural, genetic and biochemical approaches (Aim 2); and characterize the molecular events that culminate in termination of packaging using mutagenesis and biochemical analysis and cryoEM analysis (Aim 3).